1. Field of the Invention
The present invention relates in general to continuously variable traction roller transmissions, and more particularly to the transmissions of a double cavity type which has on a common axis two sets of motion transmitting traction mechanisms.
2. Description of the Prior Art
Japanese Patent First Provisional Publication 61-116166 shows one continuously variable traction roller transmission having motion transmitting traction rollers disposed between, and in engagement with, opposite toroidal discs mounted on input and output shafts. Under operation of the transmission, the torque applied to the toroidal input disc from a prime mover (for example, automotive engine) is transmitted through the traction rollers to the toroidal output disc.
During this, the rotation speed change ratio between the toroidal input and output discs is infinitely varied depending on the angle at which each traction roller inclines relative to the common axis on which the input and output discs are mounted.
In order to avoid slippage between each traction roller and the input and output discs, a biasing device is employed by which the mutually engaging roller and disc are biased toward each other. The biasing device is controlled by a controller in such a manner that the biasing force is varied in proportion to a torque applied to the input disc. Usually, a disc spring is used for the biasing device, and a loading cam structure is used for the controller.
Thus, when a large torque transmitting is required, it becomes necessary to provide the transmission with a stronger and thus large-sized biasing device. In this case, however, the entire construction of the transmission becomes large in size, particularly, in the size in radial direction.
In order to solve this drawback, there has been proposed a so-called double cavity type transmission which has on a common axis two sets of motion transmitting traction mechanisms. This type of transmission is shown in the Publication entitled "Machine Design" published on Apr. 18, 1974 and SAE (Society of Automotive Engineers, Inc.) Paper 751180. That is, by bearing the input torque by the two motion transmitting traction mechanisms, each mechanism is allowed to be constructed compact in size particularly in radial direction thereby permitting reduction in radial size of the entire construction of the transmission. As is known, reduction in size of automotive parts is quite advantageous in manufacturing of motor vehicles.
In fact, in the transmission of such type, the respective input discs of the two mechanisms are positioned adjacent to each other, and a loading cam structure is provided for each input disc. Each loading cam structure functions to bias the input disc toward the corresponding output disc in accordance with a torque applied to the input disc. The loading cam structures are supported by an intermediate member which is engaged with respective support shafts of the two mechanisms.
However, due to inherency of the above-mentioned arrangement of the loading cam structures, undesirable phenomenon tends to occur that during operation of the loading cam structures, the intermediate member is subjected to an axial displacement causing unstable placement of the input discs. When, in fact, the stable placement of the input discs is not obtained, the paired traction rollers fail to place their rotation centers at the center of the two input discs thereby excerting bad influence upon the speed change characteristics of the transmission.